Price may depend on where you live; here on the East Coast it's a soft market, and I've seen VERY clean boats go for that price. I think it's a lot of boat and easy speed for the money even in the $5K range, though. Couple more things to check; look for cracks in the comptip around the hook; check the trailing edge of the daggerboard well for wear (some is expected; too much leads to leaks and cracks); and of course check the daggerboards themselves. Rudders crack around the holes.
Keep in mind that moderate wind for the 20 is less than 15
Practice in the 12-15 range without the jib (boat tacks well w/out jib thanks to the boards), and downhaul on and travelled out. Don't come out of the gybe too high; boat is very stable dead downwind. Really depends on your tolerance or expectation of excitement...